The present invention relates to a belling machine for forming sockets on the ends of pipes made of thermoplastic material. The invention also relates to a method of forming a socket at the end of a pipe made of thermoplastic material.
In particular, the present invention relates to a belling machine and a method of forming which use specific cooling of the socket formed.
In pipes made of thermoplastic material one of the most widespread systems for making the joint between two pipes is the “socket” system. It consists of forming a socket at one of the two ends of a pipe, then inserting into the socket thus formed the end of another pipe which has not had a socket formed on it.
The sockets at the end of the pipes are made using a thermoforming process by a suitable belling machine. The belling machine is normally installed downstream of a pipe extrusion line and receives from the latter the cut pipes to be machined. Belling machines usually comprise a machining head equipped with at least one oven for heating the end of the pipe on which the socket must be formed, and a forming station which uses a suitable mould to form the heated end of the pipe into a socket. The socket is also usually cooled in the forming station. Cooling may take place simultaneously with and/or after socket forming. When the socket has reached a temperature close to ambient temperature, the socket is removed from the mould and the pipe which has already undergone the belling process is unloaded from the machine. The most widespread belling techniques use compressed air which, introduced into an environment which can be pressurised, has a fluidic action, pressing the walls of the end of the pipe, which are heated and plastically deformable, against a metal mould.
The metal moulds may form the outer shape of the socket. In this case the mould is made on the inner walls of a cavity, in which the end of the pipe on which the socket is to be formed is inserted. The compressed air is introduced into the pipe and pushes the walls of the end of the pipe from the inside towards the outside against the inner walls of the cavity. If the final shape of the socket requires the presence of an annular seat for a seal, a corresponding annular groove is made on the inner walls of the cavity which forms the mould.
The metal moulds may form the inner shape of the socket. In this case, the mould is configured like a mandrel (or plug) and in particular consists of the outer lateral walls of the mandrel. The mandrel is positioned inside a cavity which can be pressurised and is forced axially into the end of the pipe. The compressed air is introduced into the cavity which can be pressurised outside the pipe (and the mandrel) and pushes the walls of the end of the pipe from the outside towards the inside against the outer walls of the mandrel. If the final shape of the socket requires the presence of an annular seat for a seal, the mandrel is equipped with expandable sectors located in the zone corresponding to the seal seat. Once the seal seat has been formed, said expandable sectors are retracted inside the mandrel before the mandrel is removed from the end of the pipe on which the socket has been formed. Systems which use this type of mould are described, for example, in patent documents IT 1 169 179, EP 0 516 595 and EP 0 684 124.
The metal moulds may form both the inner shape and the outer shape of the socket. In this case the configuration of the mould is a combination of the two cases described above: a mandrel, whose outer lateral surfaces form the inner shape of the socket, is inserted in a cavity which can be pressurised whose inner lateral surfaces form the outer shape of the socket. The mandrel is then forced coaxially into the end of the pipe on which the socket is to be formed. Then, during a first step the compressed air is introduced into the pipe and pushes the walls of the end of the pipe from the inside towards the outside against the inner walls of the cavity. During a second step, after the space between the mandrel and the pipe has been at least partly depressurised, the compressed air is introduced into the cavity which can be pressurised outside the pipe (and the mandrel) and pushes the walls of the end of the pipe from the outside towards the inside against the outer walls of the mandrel. An example of this type of system is described in patent document EP 0 700 771.
The socket can normally be cooled by making a flow of cooling fluid directly strike the walls of the pipe and, therefore, using forced convection phenomena. Alternatively or in addition, the socket may also be cooled by removing heat from the socket by conduction through the metal walls of the mould. In particular, for the purpose of the latter, the mould may in turn be cooled by circulating cooling fluids inside it. In the case of systems which use mechanical mandrels equipped with expandable sectors for forming the seal annular seat, the presence in the mandrel of complex mechanisms dedicated to the movement of the expandable sectors makes cooling of the inside of the mandrel by circulating coolants impracticable.
A forced air flow is often used as the cooling fluid and, using a separate circuit, compressed air is used for the socket forming process. An example of this type of belling machine is found in patent document IT 1 169 179. During forming, while the compressed air is introduced into the forming chamber through a relative inlet, the forced cooling air flow is introduced into the mandrel to cool the latter. When forming is complete, the supply of compressed air is stopped and the forced cooling air flow is introduced into the chamber through a different inlet to that of the compressed air.
In an alternative embodiment of this system, described in patent document EP 0 561 594, at the end of forming, the low pressure forced air flow continues to be introduced into the mandrel, whilst the supply of compressed air is not stopped, but instead it is allowed to exit the forming chamber, so that its flow, continuously renewed, cools the outer surface of the socket just formed. The same process air used for forming the socket is therefore used to cool it.
Another example of a system in which the same process air is used both for forming and for cooling is described in patent document EP 0 684 124. In this system the only process air used is the compressed air for forming the socket. It is introduced into the forming chamber through a relative inlet and pressurises the chamber to a predetermined pressure level, forming the socket on the forming mandrel. Once the predetermined maximum pressure level has been reached and forming is complete, an outlet valve is opened, allowing the compressed air to exit the forming chamber through an outlet duct which leads into the mandrel. The inflow of compressed air is simultaneously maintained. This creates an air flow which cools the socket and which is then directly introduced into the mandrel, also helping to cool the mandrel. The air is then made to exit the mandrel through suitable outlet holes and is dispersed in the environment.
As already indicated, the systems described in patent documents IT 1 169 179, EP 0 516 595 and EP 0 684 124 (and briefly outlined above) use a mould consisting of a mandrel with a shaped exterior, inserted in a cavity which can be pressurised. During the forming step, the forming chamber is pressurised so as to minimise (or even eliminate) leaks of the compressed air introduced into it. During the cooling step after forming, part of the process air is allowed to exit the forming chamber although introduction of the process air into the chamber continues, so as to establish a flow of continuously changed air on the socket formed. The air exiting the chamber, if it does not contain too much moisture, can be collected and introduced into the mandrel to help to cool the latter (and, therefore, to cool the inner surface of the socket).
Belling machine performance, in terms of pieces produced per unit of time, is affected by the time for which the pipe remains in the forming and cooling station. The shorter the forming and cooling cycle is, the faster and more productive the machine is. The forming and cooling techniques are selected based on a suitable compromise between maximising the production speed and minimising the cost and complexity of the devices and of the components necessary to achieve the result.
The compressed air used in the socket forming and cooling processes is usually taken from the normal air distribution networks present in factories, distribution networks in which the air is usually available at a pressure of between 5 bar and 7 bar. Sometimes the air is taken directly from the environment surrounding the belling machine and is sent to the latter by a compressor suitably installed on the machine. The temperature and moisture content of the compressed air which is made available in this way are greatly affected by the environmental conditions and by the features of the compressed air generation and treatment system of the factory distribution network (or the compressor located on the machine).
It is particularly important to introduce air at a suitably low temperature into the forming and cooling devices, since the air temperature is directly correlated to the speed at which the process for cooling the socket formed takes place. In this context, air from the factory distribution network (or from the compressor located on the machine) must usually be treated, that is to say cooled, so that it reaches and stably remains at a temperature level low enough to guarantee an effective cooling process. The temperature of the compressed air obtained from the network or from the compressor is usually too high to be used directly in the socket cooling processes. Moreover, the thermal state of the compressed air obtained from the network or from the compressor is quite variable and difficult to estimate when designing the belling machine. The thermal state of the compressed air is affected by the configuration and efficiency of the factory compressor systems and by the temperature and moisture of the environment.
The normal cooling techniques for fluids involved in forming processes (in particular compressed air) usually use a refrigerating system outside the belling machine, which supplies in a closed cycle, using a recirculating pump, a water-based cooled heat exchange liquid (usually a solution of ethylene glycol in water). The cooled heat exchange liquid, arriving from the refrigerator, is circulated inside heat exchange devices designed to cool the fluids (in particular, the forced air flow and the compressed air) involved in the socket forming and cooling process. Where the configuration of the forming moulds allow and require it, the cooled heat exchange liquid arriving from the refrigerator is also circulated inside the socket forming moulds.
The conventional technique for cooling line compressed air, which is used in the socket forming and cooling process, is characterised by the use of tube bundle heat exchangers, in which there circulates the cooled heat exchange liquid arriving from and recirculated by the refrigerating system. The process air to be cooled is conveyed in these heat exchangers in such a way that it strikes the walls of the tubes in which the heat exchange liquid circulates, transferring heat to the latter.
When the cooling system to be integrated on the belling machine is being designed, it is necessary to size these refrigerators and heat exchangers in such a way that a suitable compromise is reached between the effectiveness of the cooling process obtained and the costs and dimensions of the cooling system. In order for the costs and dimensions of cooling systems to be sustainable at an industrial level, the refrigerators and heat exchangers designed end up with a series of insurmountable operating and performance limits, listed in detail below. It must first be stated that, in this context and in the description and claims, unless explicitly stated otherwise, when referring to air temperature values at a point where the air is moving at a predetermined speed and in a directed flow, reference is made to the temperature measurement obtained by inserting the sensitive element of a suitable thermometer in the flow.
It is practically impossible to obtain a compressed air temperature lower than 13° C. at the forming device inlet. Temperatures between 15° C. and 20° C. are normally obtained.
When the environmental thermal conditions and/or the temperature of the compressed air to be treated in the heat exchanger vary significantly, it is difficult to make the compressed air at the forming device inlet stably maintain a temperature below 20° C.
The conventional cooling systems described above do not allow a reduction of the moisture present in the compressed air. If the moisture in the air exceeds a predetermined threshold, problems may arise during the forming process, when the end of the pipe to be formed into a socket is very hot. In these conditions, the surface of the pipe may alter in an unacceptable way if it makes contact with the moisture in the air.